Bright polishing of cadmium and zinc



United States Patent 3,171,766 BRIGHT POLISHING 9F CADMIUM AND ZINCKenneth P. Bellinger, Ellington, Conn, assignor to Conversion ChemicalQorporation, Rockviile, Conn., a corporation of Connecticut No Drawing.Filed Oct. 4, 1962, Ser. No. 228,252 19 (Ilaims. (Cl. 156-20) Thepresent invention relates to compositions and methods for finishingsurfaces of cadmium, zinc and alloys thereof.

The term zinc surfaces as used herein refers to surfaces of zinc andzinc-base alloy articles and other metallic articles having a coating ofzinc or zinc-base alloy such as produced by hot-dipping orelectroplating.

The term cadmium surfaces as used herein refers to surfaces of cadmiumand cadmium-base alloy articles and other metallic articles having acoating of cadmium or cadmium-base alloy such as produced byelectroplating.

It is the aim of the present invention to provide a novel compositionand method for finishing zinc surfaces and cadmium surfaces which isrelatively simple in operation and provides protection therefor.

Another aim is to provide a simple and effective dilute single-dipcomposition and method for treating surfaces of zinc and zinc-basealloys to impart a bright finish thereto.

It is also an aim to provide a single-dip composition and method forsurface-treating zinc and zinc-base alloys which give a high degree ofcorrosion protection in a bright or bright iridescent finish.

A further aim is to provide a self-contained dry powder composition foraddition to water which will yield a bath for finishing zinc surfacesand cadmium surfaces to impart protection thereto.

It has now been found that the foregoing and related aims can be readilyattained by immersing the zinc or cadmium workpiece in a dilute aqueousbath containing essentially molybdate ions, nitrate ions, and anactivating ion selected from the group consisting of fluoride andsulfate, the bath having a pH of about 0.5-2.2 and preferably about0.9-1.5. A leveling agent such as borate ion or acetate ion may also beincorporated to control the bath operation and produce increasedclarity. The bath is maintained at ambient temperatures, and theworkpiece is immersed therein for a period of about five to thirtyseconds, preferably with agitation.

Generally, the dilute baths of the present invention contain essentiallyabout 0.2-6.5 grams per liter molybdate ion, 4.0-1000 grams per liter ofnitrate ion, {Ll-15.0 grams per liter of activating ion selected fromthe group consisting of fluoride and sulfate, and up to about 4.0 gramsper liter of a leveling ion selected from the group consisting of borateand acetate. The nitrate ion should be maintained at a weight ratiorelative to the molybdate ion in excess of about 10:1 to obtaindesirable brightening but less than about 40:1 for desirable corrosionprotection, and preferably on the order of 30:1.

By use of certain weight ratios of activating ion to molybdate ion, ithas been found that good brightening action upon zinc can be readilyattained through varying concentrations of the two components in thebath. When using the fluoride ion, it has been found desirable toutilize a fluoride to molybdate ion weight ratio of about 1.3-3zl, andpreferably about 1.4-1.8:1. When using the sulfate ion, it has beenfound desirable to utilize a sulfate to molybdate weight ratio of about0.5-2.0:1, and preferably about 0.7-1.2:1. Use of both sulfate andfluoride ions in a common bath formulation has been found generally toreduce the brightness of finish and, accordingly, only one of theactivating ions should be employed 3,171,765 Patented Mar. 2, 1965 foroptimum results. In practice, the fluoride ion baths appear to providegreatest brightness.

Leveling agents are desirably incorporated in the bath for pH controland control of the rate of bath action to achieve optimum brighteningwithout the need for as critical control of times and temperatures. Aslittle as 0.05 gram per liter will produce a beneficial effect, andamounts up to about one-third the weight of molybdate and activatingion, or about 4.0 grams per liter may be utilized, although thecorrosion protection will generally be reduced by increasingconcentration of the leveling ion. Accordingly, it is preferred to useonly about 0.1-0.8 gram per liter for optimum brightening and corrosionprotection. Borates and acetates have proven satisfactory as levelingagents.

The baths of the present invention have proven highly effective inproviding both brightness and corrosion protection upon Zinc surfaces,the brighter finishes surprisingly demonstrating good corrosionprotection, and the dull finishes generally exhibiting considerablylesser or no significant degree of corrosion protection as measured bythe lead acetate test. 1 Although the present invention may be utilizedto produce a bright finish upon cadmium surfaces under closelycontrolled conditions, generally there is produced a finish which variesfrom a cloudy film or yellow or blue iridescent to dull grey inappearance but which does provide some measure of corrosion protection.Accordingly, the present invention in its preferred aspect is directedto the treatment of zinc surfaces.

The several ions can be added to the bath as the acid or as salts whichare soluble in water or acid solutions. However, since molybdic acid isrelatively insoluble in both water and acid solutions, the alkali metalmolybdates and particularly sodium molybdate have been advantageouslyemployed.

Specific examples of finishing baths in accordance with the presentinvention and utilizing nitric acid are as follows:

FLUORIDE ION Range Optimum 0.25-4.13 g 1.0 g. 050- g 2.0 5:. Up to 4.0 g0.15 g. 4.5-7.5.0 ce 18.0 on. To 1.0 liter To 1.0 liter.

SULFATE ION Range Optimum Alkali Metal illolybdate 0 5-8.0 g 2.0 g.Alkali Metal Bisulfate 0.5-8.0 2.0 g. Boric Acid Up to 5.0 g 0.15 g.Nitric Acid, 42 Baum (ill-100.0 ce 24.0 cc. Water To 1.0 liter To 1.0liter.

By use of sufficient quantities of acidic salts to provide various ofthe component ions, a self-contained dry powder formulation can beprepared which will form the desired bath upon addition to water. Alkalimetal-hydrogen and ammonium-hydrogen salts of fluoride and sulfate havebeen particularly useful as has urea nitrate for this purpose.

Generally, a dry powder formulation for addition to water containsessentially about 2.0-8.0 percent by weight alkali metal molybdate;10-400 percent by weight of an A drop of an aqueous solution containing5 percent by weight lead acetate is placed upon the surface and the timenoted util the spot turns black. Generally 10 seconds is 111- dicativeof a high measure of protection since an untreated panel will turn blackimmediately.

activatin a cut havin an anion selected from the roup D b O 1 consistingof sulfates and fluorides and a positive radical selected from the groupconsisting of alkali metal-hydrogen, alkali metal and ammonium-hydrogen;up to 8.0 percent by weight of a solid leveling agent having an anionselected from the group consisting of acetate and borate and a cationselected from the group consisting of hydrogen, alkali metal andammonium; and '50.095.0 percent by weight of a nitrate compound having acation selected from the groupconsistin'g of alkali metal, urea andamrnonium, at least one of said components being sufficiently acidic innature to provide a pH of 0.52.2 upon addition of the composition towater in sufficient quantity to provide about 0.258.0 grams per liter ofalkali metal molybdate.

V Specific examples of self-contained dry powder compositions inaccordance with the present invention are as follows:

FLUO RIDE ION Either'type of dry powder formulation may be added .towater in amounts sulficient to provide the desired concentration ofmolybdate ion; i.e., about 0.26.5 grams per liter or about 025-80 gramsper liter of alkali metal molybdate. Generally, this will require about100-1000 grams of powder formulation per liter of water. 'T he pH of theresultant bath will vary with the concentration of the acidic componentwithin the powder formulation and within the bath.

Generally, the baths of the present invention are employed at ambienttemperatures of about 6590 degrees Fahrenheit and require immersion forshort periods of about five to thirty seconds, preferably with mildagitation. However, low elevated temperatures of up to about 120 degreesFahrenheit may be used, although with lesser control and shorter diptimes. The workpieces are then rinsed in cold water.

Since the baths are corrosive, suitable tanks and handling equipmentshould be employed to avoid containina:

tion such as rigid polyvinyl chloride or glass fiber reinforced resin.

Exemplary of the efiicacy of the present invention are the followingspecific examples wherein test panels 2 inches x 3 inches wereelectroplated with zinc or cadmium and immersed in the severalformulations under equivalent conditions of temperature, time andagitation.

Example 1 A bath was prepared by admixing 2.0 grams sodium acidfluoride, 1.35 grams sodium molybdate, 0.14 gram boric acid and 21.0 cc.of 42 Baum nitric acid in water to make 1.0 liter. The pH of the bathwas measured at 1.35 using a reference solution of pH 1.1.

A steel test panel 2 inches x 3 inches having an electro plated zinccoating thereon was immersed in the bath at room temperature for abouttwelve seconds with agitation and then rinsed in cold water. Theresultant finish was bright and free from iridescence. A drop of 5.0percent by weight lead acetate solution did not turn the finish blacrfor over twenty seconds, indicating a high degree of corrosionprotection.

Example 2 Example '3 A bath was prepared by dissolving in one liter ofwater 2.0 grams of sodium-hydrogen fluoride, 1.35 grams of sodiummolybdate, and 30.0 grams of urea nitrate. The resultant pH was 1.8.

A zinc-plated steel test panel Zinches x 3 inches was immersed thereinat room temperature for about twelve seconds and then rinsed. Theresultant finish was bright and gave a value of twenty seconds in thelead acetate test, indicating a high degree of corrosion protection.

Example 4 A bath was prepared similar to that in Example 3 except thatit additionally contained 10.0 grams of sodium nitrate. The pH was 1.7.A zinc-plated test panel 2 inches x 3 inches was immersed therein forabout twelve seconds with agitation and'then rinsed in cold water. Thepanel finish was bright and gave a surprisingly high test value offifty-five seconds in the lead acetate test, indicating a very highdegree of corrosion protection.

The addition of a further 10.0 grams of sodium nitrate produced a bathhaving a pH of 1.8 and resulted in a milky or cloudy finish upon thetest panel. However, the value in the lead acetate test was notdiminished.

Example 5 i A bath was prepared by dissolving in one liter of water 5.6grams of sodium bisulfate, 1.35 grams of sodium m'olybdate and 2-0.0grams of sodium nitrate. The resultant pl-l was 1.0.

A Zinc-plated steel test panel 2 inches by 3 inches was immersed thereinat room temperature with agitation for about twelve seconds and thenrinsed in cold water. The resultant finish was bright blue and gave avalue of fifteen seconds in the lead acetate test, indicating a highdegree of corrosion protection.

Example 6 A bath was prepared by dissolving 2.1 grams of sodiumbifiuoride, 1.35 grams of sodium molybdate, "0 .14 gram of boric acidand 20.0 grams of urea nitrate in one liter of water.

A cadmium-plated steel panel 2 inches x 3 inches was immersedthereinwith agitation for about ten seconds at room temperature and then rinsedin water. The resultant finish was of yellowish, slightly iridescent hueand gave a value of six seconds in'the lead acetate test, indicating afair measure of corrosion protection.

Thus, it can be seen from the foregoing detailed specification andspecific examples that the present invention provides a novelcomposition and method for finishing zinc and cadmium surfaces toprovide protection therefor which is relatively simple in operation. Thecompositions of the present invention provide a single-dip bath forsurface treating zinc andzinc-base alloys to provide a bright finishtherefor which will give a high degree of corrosion protection.According to one aspect of the present invention, it is possible toprovide a self-contained dry powder composition for addition to water toyield a Having thus described the invention, I claim:

1. A bath for treating zinc surfaces and cadmium surfaces comprising adilute aqueous acid solution containing essentially about 0.2-6.5 gramsper liter molybdate ion, 40-1000 grams per liter nitrate ion, and 01-150grams per liter of an activating ion selected from the group consistingof fluoride and sulfate, said solution having a pH of about 0.5-2.2.

2. The bath in accordance with claim 1 wherein said solution contains aleveling ion selected from the group consisting of borate and acetate inan amount of about 0.05-4.0 grams per liter and not more than one-thirdthe weight of the molybdate and activating ions.

3. The bath in accordance with claim 1 wherein the weight ratio ofnitrate ion to molybdate ion is -40: 1.

4. The bath in accordance with claim 1 wherein the activating ion isfluoride and the fluoride to molybdalte weight ratio is 1.3-3.0:1.

5. The bath on accordance with claim 1 wherein the activating ion issulfate and the sulfate to molybdate weight ratio is 0.5-2.0: 1.

6. The bath in accordance with claim 1 wherein the pH is about 0.9-1.5.

7. A bath for treating zinc surfaces comprising a dilute aqueoussolution containing essentially 0.25-40 grams per liter of alkali metalmolybdate, fluoride ion in a fluoride to molybdate weight ratio of about1.4-1.8:1, and nitrate ion in a nitrate to molybd ate weight ratio ofabout 10-40:1, said solution having a pH of about 0.9-1.5.

8. A bath for treating zinc surfaces comprising a dilute aqueoussolution containing essentially 0.5-8.0 grams per liter alkali metalmolybdate, sulfate ion in a sulfate to molybdate weight ratio of about1.5-2.0:1, and nitrate ion in a nitrate to molybdate weight ratio ofabout 10- 40:1, said solution having a pH of about 0.9-1.5.

9. A bath for treating zinc surfaces comprising a dilute aqueoussolution containing essentially 025-40 grams per liter alkali metalmolybdate, 0.5-12.0 grams per liter alkali metal-hydrogen fluoride and4.5-75.0 cc. per liter nitric acid, said bath having a fluoride ion tomolybdate ion ratio of about 1.4-1.8:1 and a nitrate ion to molybdateion ratio of about -3021, said bath having a pH of about 0.9-1.5.

10. A self-contained dry powder formulation for addition to water toprovide a bath for treating zinc surfaces and cadmium surfaces to impartcorrosion protection thereto, said formulation containing essentiallyabout 2.0-8.0 percent by weight waiter-soluble alkali metal molybdate;10-400 percent by weight of an water-soluble activing agent having ananion selected from the group consisting of sulfates and fluorides and apositive radical selected from the group consisting of alkalimetalhydrogen, alkali metal and ammonium-hydrogen; up to 8.0 percent byweight of a water-soluble solid leveling agent having an anion selectedfrom the group consisting of acetate and borate and a cation selectedfrom the group consisting of hydrogen, alkali metal and ammonium; and50.0-95.0 percent by weight of a water-soluble nitrate compound having acation selected from the group consisting of alkali metal, urea andammonium, at least one of said components being sufiiciently acidic innature to provide a pH of 0.5-2.2 upon addition of the composition towater in sufficient quantity to provide about 0.25- 8.0 grams per literof alkali metal molybdate.

11. The dry powder formulation in accordance with claim 10 wherein thepH produced by said acidic component upon addition to water is 0.9-1.5.

12. The dry powder formulation in accordance with claim 10 wherein saidnitrate compound is urea nitrate.

13. The dry powder formulation in accordance with claim 11 wherein saidnitrate compound is in an amount providing a nitrate ion to molybdateion ratio in the formulation of 10-4021.

14. The dry powder formulation in accordance with claim 10 additionallycontaining up to 8.0 percent by weight of a water-soluble solid levelingagent having an anion selected from the group consisting of acetate andborate and a cation selected from the group consisting of hydrogen,alkali metal and ammonium.

15. The dry powder formulation in accordance with claim 10 wherein theWeight ratio of nitrate ion to molybdate ion is 15-3011.

16. The method of treating zinc surfaces and cadmium surfaces to impartcorrosion protection thereto compristing providing a dilute aqueous acidsolution containing essentially about 0.2-6.5 grams per liter molybdateion, 40-1000 grams per liter nitrate ion, and 0.1-15 .0 grams per literof an activating ion selected from the group consisting of fluoride andsulfate, said solution having a pH of about 0.5-2.2; maintaining saidsolution at a temperature of about -120 degrees Fahrenheit; andimmersing therein a workpiece having a surface selected from the groupconsisting of zinc surfaces and cadmium surfaces for a period of five tothirty seconds to develop corrosion protection thereon.

17. The method in accordance with claim 16 wherein said solution has apH of 0.9-1.5.

18. The method in accordance with claim 16 wherein the weight ratio ofnitrate ion to molybdate ion is 10- 40:1.

19. The method in accordance With claim 16 wherein said workpiece has azinc surface and said solution provides a bright surface finish thereto.

References Cited in the file of this patent UNITED STATES PATENTS2,186,579 Dupernell et al Jan. 9, 1940 2,377,593 Bunte June 5, 19452,904,413 Hampel Sept. 15, 1959 2,904,414 Ostrander et al Sept. 15, 19593,072,516 Bellinger et a1 Jan. 8, 1963

16. THE METHOD OF TREATING ZINC SURFACES AND CADMIUM SURFACES TO IMPARTCORROSION PROTECTION THERETO COMPRISING PROVIDING A DILUTE AQUEOUS ACIDSOLUTION CONTAINING ESSENTIALLY ABOUT 0.2-6.5 GRAMS PER LITER MOLYBDATEION, 4.0-100.0 GRAMS PER LITER NITRATE ION, AND 0.1-15.0 GRAMS PER LITEROF AN ACTIVATING ION SELECTED FROM THE GROUP CONSISTING OF FLUORIDE ANDSULFATE, SAID SOLUTION HAVING A PH OF ABOUT 0.5-2.2; MAINTAINGING SAIDSOLUTION AT A TEMPERATURE OF ABOUT 60-120 DEGREES FAHRENHEIT; ANDIMMERSING THEREIN A WORKPIECE HAVING A SURFACE SELECTED FROM THE GROUPCONSISTING OF ZINC SURFACES AND CADMIUMN SURFACES FOR A PERIOD OF FIVETO THIRTY SECONDS TO DEVELOP CORROSION PROTECTION THEREON.